Control and monitoring systems on machines used today often include shaft speed monitors. Some of these shaft speed monitors merely monitor the speed and let a machine operator know the shaft speed. If the speed is incorrect, the operator can make required adjustments. Other shaft speed monitors send a signal proportion to the measured shaft speed, to a controller. The controller receives the shaft speed signal, processes the signal and generates a response if required. This response could be to change some function of the machine or to send a warning to an operator.
Shaft speed monitors currently used include a toothed wheel mounted on a rotating member and a signal generator mounted close to the toothed wheel. The signal generator as well as the toothed wheel have been relatively unprotected and exposed to the elements. Dust, dirt and foreign matter can degrade the signal generated by the shaft speed monitor. Water can also degrade the signal generated. Foreign objects such as small rocks can knock the sensor assembly out of alignment with the toothed ring and render the shaft speed monitor inoperable